In antenna systems, such as satellite antenna systems used, for example, in a global positioning system (GPS) or in a communications system, a size of a produced beam is selected to cover a particular country or a geographic area. A beam size can be varied to increase or reduce a covered area.
For example, U.S. Pat. No. 6,243,051 discloses a dual helical antenna for a GPS including a reflector and a focal point. Two multi-turn axial mode helical antenna elements are arranged on a support shaft extending axially from the reflector to the focal point. One of the helical antenna elements is disposed at the focal point, and the other antenna element is disposed at a defocused position to broaden the beam and covered area.
However, this antenna system is constrained to large and small beams only, and provides no medium or middle beam size setting because a beam size cannot be continuously varied. Also, for multi-frequency applications, such as a GPS, the relative beamwidths are dependent and constrained to be proportional to frequency. In addition, power handling capability of the system is limited by a single feed element.
Another example of an antenna system with a variable-size beam is presented in U.S. Pat. No. 6,577,282 that discloses a system for zooming and reconfiguring circular beams. The system includes a feed horn, a subreflector, a main reflector, and a connecting structure. The feed horn is pointed at an axis removed from the bisector axis of the subreflector. A size of the produced beam is changed by changing the distance between the feed horn and the subreflector.
This system changes a beam size mechanically. The system requires a moving mechanism for changing the distance between the feed horn and the subreflector. Such a mechanism reduces reliability and increases weight of the system. Further, for multi-frequency applications, the relative beamwidths are dependent and constrained to be proportional to frequency. Moreover, the system is restricted to beams of a circular nature.
A system for electronically controlling a beam size is disclosed in U.S. Pat. No. 5,151,706. This system includes an array of N radiating elements subdivided into P subarrays of M elements each, a common signal source, a power divider that distributes the signal delivered by the source, amplifiers, and means for selectively exciting some of the elements with the amplified signal at a controlled phase shift so as to obtain a desired radiation pattern.
There are several significant drawbacks to this approach. First, the total power that can be directed to any one output is only a fraction of the total amplifier power, because the power divider is segmented corresponding to subarrays each driven by only a subset of the power amplifiers. Further, this concept is limited to arrays, which can be properly excited when the power from each coupler is directed into elements which are uniformly interleaved with elements driven from the other couplers. This element interleaving constraint is necessary to work within the limitation of the subarray couplers, which is that the input power to any coupler can only be directed into a single output or into two outputs independently. Power cannot be directed to 3 coupler outputs, and when power is directed to 4 outputs from any given coupler, the amplitudes cannot be controlled independently. Moreover, this concept is limited to excitation of linear or radial arrays and does not allow a beam to be varied in two dimensions.